An international team including scientists from Johannes Gutenberg University Mainz (JGU) has published a reconstruction of the climate in northern Europe over the last 2,000 years based on the information provided by density measurements of sub-fossil pine trees originating from Finnish Lapland. Professor Dr. Jan Esper’s group at the Institute of Geography at JGU’s measurements produce a reconstruction reaching back to 138 BC.

In these reconstructions, the researchers have been able, for the first time, to precisely demonstrate that the long-term trend over the past two millennia has been towards climatic cooling. “We found that previous estimates of historical temperatures during the Roman era and the Middle Ages were too low,” said Professor Dr. Jan Esper. “Such findings are also significant with regard to climate policy, as they will influence the way today’s climate changes are seen in context of historical warm periods.” The new study has been published in the journal Nature Climate Change.

Was the climate during the Roman era and Medieval times warmer than today? A study suggests that the Britain of 2000 years ago experienced a lengthy period of hotter summers than today. We have records in literature of grapes growing in northern England. These are the questions that the discipline of paleoclimatology tries to answer.

Why are earlier warm periods important when what we are interested in today is current global climate changes? Thermometer records are recent. Scientists analyze indirect evidence of climate variability, such as ice cores and ocean sediments, and so reconstruct the climate of the past. The annual growth rings in trees are the best witnesses we have over the past 1,000 to 2,000 years as they indicate how warm and how cool past climate conditions were.

In the cold environment of the Nordic taiga in Finnish Lapland, trees often collapse into one of the numerous lakes, where they remain well-preserved for thousands of years. The density measurements correlate closely with the summer temperatures in this cold area. Researchers were able to create a temperature reconstruction of unprecedented quality. It provides a high-resolution representation of temperature patterns in the Roman and Medieval Warm periods and also show the cold phases that occurred during the Migration Period and the later Little Ice Age.

The new climate curve exhibited an unexpected phenomenon in this form. For the first time, researchers were able to use the tree-ring data to calculate precisely a much longer-term cooling trend that has been playing out over the past 2000 years. This trend involved a cooling of -0.3° per millennium due to gradual changes to the position of the sun and an increase in the distance between the Earth and the sun.

Dr. Esper said “This figure we calculate may not seem particularly significant. However, it is also not negligible when compared to global warming, which up to now has been less than 1°C. Our results suggest that the large-scale climate reconstruction shown by the Intergovernmental Panel on Climate Change (IPCC) likely underestimate this long-term cooling trend over the past few millennia.” (click to enlarge)